Tolerances, Rework and Who Pays for It

Strip the shuttering and put a tape on the column: 470 mm against 450 mm on the drawing. The shuttering thekedar shrugs — "concrete hai, thoda aage-peeche chalta hai." The client's engineer wants it demolished. Both are wrong, and the referee between them is a number neither has read: the tolerance. Construction is not machining; no cast element lands exactly on the drawing. Codes and specifications therefore define how much deviation is acceptable — and everything about money follows from which side of that line the tape lands on.

What a tolerance is — and is not

A tolerance is the deviation a specification permits without consequence. Work inside tolerance is compliant: it cannot be rejected, and it earns no deduction. Work outside tolerance is non-conforming: it triggers review, rework or recorded acceptance — never silent continuation. Two things a tolerance is not:

• Not a free allowance to aim for. The target is the drawing dimension; the tolerance absorbs honest construction variation, not planned undersizing of every member by the permitted minus.
• Not a payment bonus. Work at +12 mm is still paid at drawing dimensions (Lesson 1's payment line). Tolerance decides acceptability, the drawing decides payment.

The numbers that matter on an RCC site

IS 456 (clause 11.1) sets the formwork/dimension tolerances for concrete work:

Note the asymmetry: the code forgives extra size far more than undersize, because undersize eats into designed strength and cover. Alongside these:

• Nominal cover: actual cover may exceed nominal by up to 10 mm but must never be less — undersized cover is a durability defect, not a tolerance question.
• Delivered slump: ±25 mm or one-third of the specified value, whichever is less (IS 4926, Lesson 3).
• Finishing tolerances are contractual, not from IS 456. Plaster and floor flatness are commonly specified around 3–4 mm under a 2 m straightedge in good-practice specs — check your contract, because unwritten tolerance means unwinnable argument. And match the spec to the walling material: AAC block walls (now common in metro and western/southern markets) come true enough for 10–12 mm plaster, while irregular clay brick — still the norm across much of the North and East — often needs 15–20 mm specified. A 12 mm plaster spec on wavy brick is a tolerance dispute pre-ordered.

So the verdict on our opening column: 470 against 450 is +20 mm — beyond the +12 permitted. Not demolition by default, but not a shrug either: it goes down the non-conformance route below.

Worked example 1: the bulge nobody pays for

Even within tolerance, deviation costs the executing side. A floor has 58 m of beams, 230 mm wide, poured against tired shuttering that bulged an average 15 mm per side. Extra width 30 mm, web depth below slab 325 mm:

The client pays 230 mm wide beams — the drawing — so this concrete is free to the client and a straight loss to the concrete contractor, floor after floor. The concrete contractor in turn recovers it from his shuttering thekedar (whose sqft-rate team owns formwork stiffness) only if the deviation was measured and recorded when the shutters came off. No record, no recovery: the loss quietly becomes "consumption."

Beyond tolerance: the NCR route

When the tape lands outside the band, the disciplined path is a Non-Conformance Report (NCR): a numbered record of what deviates, by how much, where — routed to the designer/client for a decision. The decision space is fixed:

1. Accept as-is or with review — common for oversize (extra concrete rarely harms strength; the designer confirms weight and architectural fit). Acceptance is recorded, often with the extra unpaid and any knock-on costs (fouled tile lines, reduced clear openings) to the executing party.
2. Accept with deduction — the contract's mechanism (or a negotiated rate reduction) applies; the NCR plus MB note is what makes the deduction stick at final bill and survive arbitration.
3. Rectify or recast — mandatory when undersize or misplacement threatens strength, cover or function.

Worked example 2: the undersized column

A 230 x 450 column strips at 230 x 438 — minus 12 mm, double the permitted −6. The structural designer reviews: at this floor the column has margin, but cover to the main bars is now below nominal on one face. Indicative mid-2026 costs of the three roads (one column, one floor lift):

Every rupee of whichever road is chosen sits on the executing party's account — this is the "who pays" rule in one line: the party whose workmanship left tolerance pays for the journey back inside it. The client's exposure is time, which is why prevention (shutter checks before pour — Lesson 2's checklist) beats every cure.

Who pays — the summary logic

The RERA era adds a long tail to this table: a developer who "accepts" out-of-tolerance work without record still owes allottees defect rectification for years after possession (commonly cited as five years for structural and workmanship defects — confirm your state's rules). An NCR register that shows deviations caught, reviewed and closed is what separates a defensible handover file from an open-ended liability.

How this protects your money

For the owner/client QS: measure key elements at shutter-strike (a 10-minute tape routine per floor), because a deduction claimed at final bill without a contemporaneous record is just a haggle — the NCR dated the week of casting is what makes it enforceable. For the contractor QS: the same records cap your liability. A signed NCR closed as "accepted with review" is your proof, three years later, that the deviation was disclosed and resolved — without it, every crack in the building becomes negotiable against your retention money. Tolerances are not just engineering hygiene; they are the pricing rules for imperfection, and both sides bill better knowing them cold.

Common mistakes

• Demanding demolition for any deviation — or accepting any deviation with a shrug. Both ignore the published bands.
• Measuring only what looks wrong: bulges hide in beams nobody tapes. Routine strike-measurement, not suspicion, finds them.
• Treating tolerance as a design allowance and shuttering every member to the permitted minus.
• Verbal acceptance of out-of-tolerance work — the unpaid extra and the unrecorded acceptance both come back as disputes.
• Forgetting cover: a column within +12/−6 can still be non-conforming if bars sit at 15 mm against a 40 mm nominal cover.

What comes next

Everything so far protects money against error and dispute. The final lesson of this module prices the item most estimates pretend is free: safety — what it actually costs, where it sits in the BOQ, and why an unpriced safety item is a risk you have silently agreed to carry.